Pregnancy has been termed "Nature's transplant" because the developing fetus, essentially a foreign tissue graft, is protected from rejection by its host, the mother (Editorial: "Nature's transplant" Lancet 1:345-346, 1974). Rejection of a transplanted allograft in an immunocompetent host is normally mediated by the macrophage-derived cytokine tumor necrosis factor (TNF) (Eason et al., Transplantation 59:300-305, 1995). Excessive production of TNF during pregnancy causes spontaneous abortion (Shaarawy et al., Acta Obstet. Gynecol. Scand. 76:205-211, 1997; and Mallmann et al., Arch. Gynecol. Obstet. 249:73-78, 1991). Recently, spermine, a ubiquitous biogenic amine present in large amounts in the amnion, has been shown to counter-regulate the immune response by inhibiting the production of TNF and other pro-inflammatory cytokines by human mononuclear cells (Zhang et al., J Exp. Med. 185:1759-1768, 1997).
Fetuin is a globular 341-amino acid protein containing 20-25% carbohydrate (by weight) and 6 internal disulfide bonds. The human fetuin sequence (also known as .alpha.2-HS glycoprotein) is provided herein as SEQ ID NO. 1 and SEQ ID NO. 2. Fetuin was first identified over 50 years ago as a major protein component of bovine fetal serum but its biological function remains unclear. Bovine fetuin is a globular 341 amino acid polypeptide with six internal disulfide bonds and three N-linked and two O-linked oligosaccharide chains. Primary amino acid sequence and the position of cysteine residues are well conserved in human, bovine, sheep, rat and mouse fetuin homologs (Dziegielewska et al., J Biol. Chem. 265:4354, 1990; Rauth et al., Eur. J Biochem. 205:321,1992; Lee et al., Proc. NatL. Acad. Sci. USA 84:4403, 1987; and Brown et al., Eur. J Biochem. 205:321, 1992). Fetuin levels in human plasma are regulated in the manner of a negative acute phase reactant (Lebreton et al., J Clin. Invest. 64:1118, 1979). IL-1 was shown to suppress fetuin transcript levels in cultured hepatocytes (Akhoundi et al., J Biol. Chem. 268:15925, 1994). Fetuin appears to be expressed in bone because transcripts have been detected in both chondrocytes and osteoblasts (Yang et al., Blood 12:7, 1991). The polypeptide .alpha.2-HS glycoprotein is a human homolog of fetuin and is secreted in high levels by adult liver into the peripheral circulation (Triffitt et al., Nature 262:226, 1976).
Human fetuin has 3 N-linked oligosaccharide chains (attached to the amine nitrogen atom of asparagine), and 2 O-linked oligosaccharide chains (attached to the oxygen atom of serine or threonine). The sugar moiety directly attached to the fetuin polypeptide is usually a N-acetylglucosamine residue. The terminal sugar residue is usually a sialic acid, in particular a N-acetylneuraminic acid (NANA) residue, which bears a net negative charge. If one removes the terminal sialic acid residue from fetuin by neuraminidase treatment, the resulting glycoprotein is an asialofetuin. Fetuin is also a carrier protein for growth factors. Fetuin is sometimes referred to as .alpha.2-HS-glycoprotein. Thus, it is considered that fetuin's biological effects on cultured cells are related to its carrier function for molecules with growth-promoting properties.
The synthesis of human .alpha.2-HS-glycoprotein is down-regulated by cytokines (hIL-1.beta., hIL-6) (Lebreton et al., J Clin. Invest. 64:1118-1129, 1979). Human fetuin levels are decreased (25-50%) in trauma patients (van Oss et al., J Trauma 15:451, 1975). Therefore, there is a need in the art to find a utility for fetuin and to understand fetuin's physiological role and the importance of its many negatively charged (at physiologic pH) sialic acid residues.